On the contrary, Izuna seemed to actually respect Madara, even moreso when Izuna couldn't believe that there was somebody even stronger than Madara. Izuna was the same person that sparred with Tobirama and not even once did Izuna see Tobirama as Madara's superior, which implies that Tobirama himself wasn't on the same level as Hashirama who was the only one considered Madara's superior until Madara awakened the Sharingan. Hell, we have Hashirama's father expressing concern about Madara because he managed to kill Adults within the Senju clan, being completely aware of Madara having a brother. Madara was perceived as the greater threat, displayed more talent overall, and even awakened the Sharingan far earlier than Izuna did.
Izuna was never on Madara's level.